1. Field of the Invention
The present invention is directed to a nuclear magnetic tomography apparatus having a basic field magnet into which at least one gradient coil and one radiofrequency antenna are built.
2. Description of the Prior Art
As is known, a basic field magnet, which generates a magnetic field with which nuclear spins in an examination region are oriented in a specific direction, is required in a nuclear magnetic resonance tomography apparatus. Electromagnetic energy is emitted onto the examination subject with a radiofrequency antenna causing excursion of the nuclear spins onto out of this direction and the nuclear magnetic resonance signal arising as a result is received--possibly with the same radiofrequency antenna. A simple example of such a radiofrequency antenna is disclosed in U.S. Pat. No. 4,506,224. The radiofrequency antenna disclosed therein is composed of individual rods with their ends connected to ground via capacitors.
Magnetic gradient fields are required in an imaging apparatus for location-coding of the nuclear magnetic resonance signals, these magnetic gradient fields being generated for each spatial direction (for example, along the x,y,z axes of a Cartesian coordinate system) by a set of gradient coils. U.S. Pat. No. 4,486,711 discloses a simple example of such gradient coils.
As schematically shown in FIG. 1 herein, a superconductive basic field magnet has a cylindrical opening 1a which forms the examination space. In conventional systems, the volume within cylindrical opening 1a, is constricted by the gradient coils 3 and the radiofrequency antenna 2, which are to be applied inside the opening 1a. The diameter the cylindrical opening 1a is therefore correspondingly larger than that of the actual patient volume available for the examination. Since the latter should not fall below a certain minimum dimension in view of comfort for the patient, the size of the patient-receiving volume is defined by the fixed cylindrical opening 1a and the thickness of the radiofrequency antenna and the gradient coils. Further elements that are built into the basic field magnet, for example shim elements for improving the uniformity of the basic magnetic field, are not shown in FIG. 1 for clarity.
The outlay for the magnet becomes higher as the cylindrical opening becomes larger.
The above discussion also applies analogously to systems employing pole shoe magnets, where in the radiofrequency antenna and gradient coils are applied onto the pole shoes and the spacing between the pole shoes is determined by the height of the examination space minus the thickness of the radiofrequency the gradient coils.